As electronic apparatuses operate at higher frequencies, capacitors used in the apparatuses are demanded to have a lower equivalent series resistance (ESR), excellent impedance characteristics at the high frequencies, and large capacitance. To meet these demands, solid electrolytic capacitors employing solid electrolyte made of conductive polymer having higher electric conductivity have been developed.
A roll-type solid electrolytic capacitor has been in market to meet the demand of a large capacitance. This capacitor includes an anode foil, a cathode foil, and a separator which are wound together. This solid electrolytic capacitor is more excellent not only in its life time and temperature characteristics but also in high-frequency characteristics than other capacitors, accordingly being used in power supplies for personal computers. Patent Document 1 discloses a conventional roll-type solid electrolytic capacitor includes a cathode foil made of non-valve metal foil, such as nickel foil, which hardly has natural oxide layer thereon in order to increase a capacitance of the capacitor. This structure increases the electrostatic capacitance generated at the cathode foil to a substantially infinite value.
The non-valve metal foil disclosed in Patent Document 1 cannot be roughened by an etching process, and hence, can hardly increase an effective contact area between the cathode foil and the solid electrolyte. Thus, the solid electrolytic capacitor disclosed in Patent Document 1 is prevented from having a small ESR. Further, the nickel foil is more expensive than aluminum foil.
Patent Document 2 discloses another solid electrolytic capacitor including the cathode foil made of aluminum foil in order to overcome the foregoing problems. The surface of the aluminum foil can be roughened by an etching process, and can have a plated nickel layer, non-valve metal, formed on the roughened surface by a non-electrolytic plating method.
However, the nickel plated layer can hardly be formed uniformly inside pores formed in the roughed surface of the aluminum foil. Further, the aluminum foil can hardly be bonded to the nickel plated layer securely.
Solid electrolytic capacitors connected to CPUs of personal computers are demanded to have not only a large capacitance and a low ESR, but also a lower equivalent series inductance (ESL) even at high frequencies in order to obtain excellent noise reduction and transient response.
FIG. 23 is a perspective view of further conventional solid electrolytic capacitor 501 disclosed in Patent Document 3. FIG. 24 is a plan view of capacitor element 211 of capacitor 501. Capacitor element 211 includes an anode body made of aluminum foil, which is valve metal, a dielectric oxide layer on a surface of the anode body, insulating resist 212 for separating the surface of the anode body into anode electrode section 213 and a cathode forming section, and cathode electrode section 214 on the dielectric oxide layer at the cathode forming section. The surface of the anode body is roughened. Cathode electrode section 214 includes a solid electrolytic layer made of conductive polymer on the dielectric oxide layer at the cathode forming section, and a cathode layer on the solid electrolytic layer. The cathode layer includes a carbon layer formed on the solid electrolytic layer and a silver paste layer formed on the carbon layer. Anode electrode section 213 and cathode electrode section 214 are arranged along a longitudinal direction of capacitor element 211 while resist 212 is placed between sections 213 and 214.
Plural capacitor elements 211 are stacked on anode common terminal 215 connected to anode electrode sections 213 of capacitor elements 211. Anode electrode sections 213 of capacitor elements 211 are jointed to anode common terminal 215 by laser welding.
Cathode common terminal 216 is coupled to cathode electrode sections 214 of plural capacitor elements 211. Bend 216A is formed by bending both side ends of terminal 216 upward. Cathode common terminal 216 is bonded and connected electrically to cathode electrode sections 214 of plural capacitor elements 211 with conductive adhesive. Bends 216A are bonded and connected electrically to cathode electrode sections 214 with conductive adhesive 217.
Insulating resin package 218 covers plural capacitor elements 211 to expose respective portions of anode common terminal 215 and cathode common terminal 216 from the package. The portions exposed from resin package 218 are bent downward along resin package 218, thus constituting an anode terminal and a cathode terminal on a lower surface of resin package 218.
Bends 216A of cathode common terminal 216 are bonded to cathode electrode sections 214 of capacitor elements 211 with conductive adhesive 217 in solid electrolytic capacitor 501. This structure reduces an overall resistance of capacitor elements 211 stacked together, thus reducing the ESR of capacitor 501.
The surface of the anode body made of aluminum foil is roughened by an etching process to increase the surface area of the anode body for providing the capacitor with a large capacitance. However, technical matters of the etching process and the mechanical strength of the aluminum foil restrict the increase of the surface area of the anode body, hence preventing capacitor 501 from having a large greater capacitance.
The carbon layer or the silver paste layer constituting the cathode layer tends to have a thickness with variations, and can increase the overall resistance of the cathode layer or a contact resistance between the solid electrolytic layer and the carbon layer or between the carbon layer and the silver paste layer. As the number of capacitor elements 211 increases, the amounts of expensive materials, such as the silver paste and the conductive adhesive, accordingly making capacitor 501 expensive.    Patent Document 1: JP04-7086A    Patent Document 2: JP3439064B    Patent Document 3: JP2003-45753A